In the preparation of silver halide emulsions, fluctuations occur which result in differences in minimum densities in the processed colour photographic material. The mass production of copies, however, requires compatible photographic materials. The differing photographic materials can be compared with one another on the basis of the characteristic colour density curves of the three partial colour images. For this purpose, the photographic material is exposed through a grey wedge with a specified intensity of illumination. After colour development, the photographic material on which the step wedge has been copied is measured in a densitometer behind a blue, green or red filter.
The characteristic colour density curves of the individual partial colour images is obtained by plotting the colour density value measurements against the logarithm of exposure. These curves should take a parallel course for neutral colour reproduction. Further, when a neutral object of medium brightness is photographed, the density differences between the yellow and the magenta partial colour image and between the magenta and the cyan partial colour image obtained from different photographic materials should as far as possible be equal if comparable images are to be obtained in the copying process since copying may then be carried out with the same storage adjustment of the print systems so that it becomes unnecessary to select different photographic materials.
The final adjustment of the colour density curves of the individual partial colour images and hence also the density differences between them is carried out with so-called correction dyes chosen according to the charge of the emulsion. In order to obtain optimum image results in the region of over exposure as well as the region of under-exposure and prevent colour casts, the correction dye must be comparable to the dye produced by chromogenic development.
It has been found in practice, however, that the cyan correction dyes conventionally used, which are obtained latent image in storage under tropical conditions (e.g. at 90% relative humidity and 35.degree. C.).